Method for the treatment of waste gas from a cellulose plant

ABSTRACT

The invention relates to a method and device for the treatment of waste gas from a cellulose production, in particular, for treatment of the waste gases from a bleaching reactor ( 1 ), whereby aerosols in the waste gases are removed ( 4 ) and gaseous contaminants are scrubbed from the waste gas by means of a scrubbing fluid.

The invention relates to a process for cleaning waste gas from pulpproduction and a device for bleaching pulp in a bleaching reactor.

Based on the increased requirements of environmental protection, thepulp and paper industry has moved to using ozone or a gas mixture ofoxygen and ozone as the bleaching agent instead of chlorine.

Here, oxygen is supplied to an ozone generator, in which the oxygen ispartially converted into ozone. The resulting oxygen-ozone mixture isthen supplied in the required amount and concentration to the so-calledZ-stage of pulp production in which the pulp is bleached by ozone.

In the reaction of the pulp with the oxygen-ozone mixture, reactiongases form that are removed jointly with the residual ozone from theZ-stage and are routed via a so-called fiber scrubber in which entrainedpulp fibers are removed from the gas flow. The gas flow is then suppliedto an ozone destruction system in which the residual ozone is convertedinto oxygen.

During operation of these systems, it has been ascertained thatcorrosion occurs on numerous system and equipment parts that come intocontact with the waste gas, and on the pipelines between the Z-stage andthe ozone destruction system. The corrosion is especially serious whenthe pulp is treated not only with ozone, but with mixtures of bleachingagents, for example ozone and chlorine dioxide, or even in successionwith different bleaching agents. In mixed operations and in alltransition states between defined operations of the bleaching stage,additional aggressive substances that cause corrosion have been found inthe gas flow that has been withdrawn from the bleaching stage.

The object of this invention is therefore to develop a process and adevice for cleaning waste gas from pulp production or for bleaching ofpulp, corrosive damage on the system parts that come into contact withthe waste gas being avoided.

This object is achieved by a process of the initially mentioned type,aerosols located in the waste gas being removed and gaseous impuritiesbeing scrubbed out of the waste gas by means of a scrubbing liquid.

The device according to the invention for pulp bleaching in a bleachingreactor is characterized in that the bleaching reactor is connected viaa waste gas line to a device for removing aerosols from a gas flow and ascrubbing column for removing gaseous impurities from a gas flow.

In extensive research preceding the invention, it was found that in thereaction of pulp with a bleaching agent, especially with ozone, reactiongases of a special type are formed. The composition of these reactiongases that are formed in pulp production or processing under the mostvaried process conditions and that must be discharged as waste gases hasbeen thoroughly analyzed.

Based on this research, it was recognized that in the bleaching reactorin the reaction of the pulp with the bleaching agent, aerosols form inwhich various acids are bonded. Other aggressive components wereascertained as gaseous traces in the waste gas.

This mixture of the most varied acids and other corrosive materials iscontinuously removed with the waste gas from the pulp productionsection, then attacks the system components that come into contact withthe waste gas, and leads to corrosive damage. In the known systems, inthe past only a fiber scrubber was used for waste gas cleaning. Thelatter was able to scrub pulp fibers out of the waste gas, butaggressive substances remain in the waste gas or can only beinadequately removed from it.

The invention now makes available a cleaning process tailored to thewaste gases that are formed in the paper and pulp industry, in which theacid-laden aerosols are removed from the waste gases and the gaseousimpurities are scrubbed out of the waste gas by means of a scrubbingliquid. The invention is of great importance especially in theproduction of pulp using HC technology with ozone.

The aerosols are removed preferably with a Venturi scrubber. Theaerosol-laden waste gas is delivered into a Venturi scrubber and broughtinto contact with a scrubbing water flow that compared to the waste gasstream has a high relative speed. Due to their inertia, the aerosols inthe waste gas strike the droplets of scrubbing water, settle on them andare thus scrubbed out of the waste gas. Under certain circumstances, itcan also be quite advantageous to use a rotary scrubber instead of theVenturi scrubber. Furthermore, to remove aerosols, fabric filters,electric filters and binary-jet systems have proven favorable.

Preferably the Venturi scrubber is equipped with an adjustable nozzlecross-section. By changing the nozzle cross-section at a constant amountof waste gas, the degree of separation, i.e., the cleaning action, canbe changed. Conversely, when the amounts of waste gas fluctuate, thedegree of separation can be kept constant by adapting the nozzlecross-section. The invention thus always allows a constant cleaningaction even when the load of the pulp production system changes.

The gaseous impurities in the waste gas flow are preferably scrubbed outin a scrubbing column in which the waste gas and a scrubbing liquid arerouted in counterflow. It has been shown that a packed column isespecially well suited to scrubbing chlorine and chlorine dioxide thatcontribute to the formation of the chloride that causes corrosion. Theuse of plate columns, packed columns or bubble columns, however, is alsopossible.

Preferably, the aerosols are removed from the waste gas upstream fromthe scrubbing column. First, therefore, the aerosols are removed fromthe waste gas flow, and then the gaseous impurities are scrubbed out inthe scrubbing column. Solids in the waste gas, for example pulp fibers,are especially preferably removed from the waste gas before removing theaerosols. On the other hand, there is the danger that the device forremoving the aerosols and the scrubbing column for removing the gaseousimpurities will be clogged by pulp fibers and other particles.

The scrubbing liquid from the scrubbing column is also advantageouslyused as scrubbing water for the Venturi scrubber. In this case, ascrubbing liquid circuit is installed between the scrubbing column andthe Venturi scrubber.

Since depending on the execution of the scrubbing column and thecomposition of the waste gas, in part also ozone is dissolved in thescrubbing liquid, the scrubbing liquid is routed via an ozonedestruction unit, preferably via a catalytic ozone destruction unit,before it is removed.

By means of the invention, chemically aggressive substances are removedfrom the waste gas flow after passing through the Venturi scrubber orthe device for removing aerosols, and the scrubbing column so thatcorrosion on the system parts downstream from the scrubbing column nolonger occurs. In order to prevent corrosion in the waste gas cleaningpart, i.e., in the scrubbing column and the device for removingaerosols, they are preferably made of a molybdenum-containing austeniticsteel or a titanium alloy. These materials have proven relativelycorrosion-resistant, mainly against chloride, in tests. In particular,the steels labelled with material number 1.4539 or 1.4529 according tothe German Industrial Standard, and titanium alloys, especially titaniumalloy 3.7235 according to DIN 17851, have proven especially effective.

The process according to the invention is especially suited fortreatment of the reaction gases that form during bleaching of pulp andis used preferably when the pulp is being bleached with ozone and/orchlorine dioxide. Especially when bleaching with two different bleachingagents, whereby the bleaching agents can be used at the same time orelse in succession, it has been found that the waste gas is mixed withespecially aggressive substances. The invention can be used especiallyadvantageously in these cases.

The invention and other details of the invention are explained in moredetail below using the embodiment shown in the drawings. Here, thesingle FIGURE shows

-   -   the process diagram of a system according to the invention for        cleaning waste gas from a    -   bleaching reactor.

In a pulp and paper mill, pulp is bleached in a bleaching reactor 1 withozone and chlorine dioxide. In the chemical reactions that proceed inthe bleaching reactor 1, numerous aggressive substances form. The acidsthat form in this process are bonded predominantly in aerosols thatlikewise form in the bleaching reactor 1. Other chemically aggressivesubstances, especially molecular chlorine and chlorine dioxide, arepresent in the gaseous phase.

The waste gases that form during bleaching are continuously suctionedoff via a waste gas line 2 and are supplied to a fiber scrubber 3. Inthe fiber scrubber 3, pulp fibers that have been entrained with thewaste gas flow are scrubbed out by the water flowing in the oppositedirection. The aerosols located in the waste gas are conversely notscrubbed out in the fiber scrubber 3, but rather travel with the wastegas flow into the following system parts.

The waste gas from which the pulp fibers have been removed is thendelivered to a Venturi scrubber 4 in which the aerosols contained in thewaste gas are removed from the latter. In this process step, aggressiveacids, such as HCl, H₂SO₄ and HNO₃, and other acids that are found inthe waste gas bonded in the aerosols are removed from the waste gasflow. The Venturi scrubber 4 has an adjustable Venturi channel so thatregardless of the amount of waste gas that forms, the desired degree ofseparation of aerosols from the waste gas flow can always be ensured.Depending on the pulp flow rate through the pulp production system andthus depending on the necessary flow rate of the bleaching agent throughthe bleaching reactor 1, optimum separation of the aerosols from thewaste gas flow can always be set.

Then, the waste gas flow is supplied to a packed column 5 in which thegaseous impurities are scrubbed out in counterflow with the scrubbingliquid. The necessary amount of scrubbing liquid is supplied at the headof the column 5 via a liquid-gravity distributor in order to minimizeentrainment of drops with the gas flow. The waste water that formsduring scrubbing in the column 5 is withdrawn at the bottom of thecolumn 5. When the gaseous impurities are being scrubbed out, in mostcases also part of the ozone is dissolved in the scrubbing liquid. Insome countries, for reasons of environmental protection, ozone is notallowed to escape into the atmosphere. For this reason, the scrubbingliquid, before it is drained, for example, into the sewer system, isrouted via an ozone destruction unit that is not shown in the FIGURE inorder to destroy the ozone dissolved in the scrubbing liquid.

The scrubbing water used in the packed column 5 is removed from thebottom of the column 5, in part routed via the line 10 to the Venturiscrubber, and used there as propellant water or scrubbing water in orderto then be delivered again to the packed column 5.

In the scrubbing column 5, at least 97% of the chlorine and chlorinedioxide contained in the waste gas are scrubbed out. The otheraggressive substances are essentially completely removed from the wastegas so that the waste gas is cleaned after passing through the Venturiscrubber 4 and the scrubbing column 5.

The scrubbed waste gas that always still contains most of the residualozone originating from the bleaching reactor 1, but otherwise no longercontains chemically aggressive substances, is then supplied to acatalytic ozone destruction unit 6 that converts the residual ozone intooxygen.

The gas originating from the catalytic ozone destruction unit 6 issupplied via a cooler 7 to a negative pressure fan 8. The negativepressure fan 8, on the one hand, controls the negative pressure in thebleaching reactor 1, and on the other hand, it produces the overpressurethat is necessary for the following system. Following the negativepressure fan 8, the waste gas is again run via a cooler 9 and is then tobe compressed in an oxygen compressor that is not shown in the drawingsto the final pressure necessary for the following process steps.

1. A process comprising the steps of bleaching pulp with a bleaching gascomprising ozone; withdrawing a waste gas containing aerosols from saidbleaching step; passing said waste gas through a Venturi scrubberoperated with scrubbing water so as to scrub out the aerosols containedin the waste gas, and passing the waste gas to a different scrubbingstep so as to scrub out gaseous impurities from the waste gas by meansof a scrubbing liquid (5).
 2. A process according to claim 1, whereinthe nozzle cross section of the Venturi scrubber (4) is adjustable, andfurther comprising adjusting the nozzle cross section so as to match theamount of waste gas to be cleaned to the desired degree of cleaning. 3.A process according to claim 1, wherein in said different scrubbingstep, the gaseous impurities are scrubbed out in a scrubbing column (5).4. A process according to claim 1, wherein the gaseous impurities arescrubbed out in a packed column (5).
 5. A process according to claim 1,further comprising withdrawing some of the scrubbing liquid (10) fromsaid further scrubbing step and passing said scrubbing liquid asscrubbing water to the Venturi scrubber (4).
 6. A process according toclaim 1, wherein the bleaching gas comprises ozone and chlorine dioxide.7. A process according to claim 6, further comprising destroying theozone in the resultant scrubbed waste gas.
 8. A process according toclaim 7, wherein the ozone is catalytically (6) destroyed.
 9. A processaccording to claim 1, wherein pulp fibers in the waste gas are removedfrom the waste gas before the aerosols are removed.
 10. A processaccording to claim 1, wherein first the aerosols in the waste gas areremoved, and then the gaseous impurities are scrubbed out of the wastegas by means of a scrubbing liquid.
 11. A process according to claim 2,further comprising withdrawing some of the scrubbing liquid (10) fromsaid different scrubbing step and passing said scrubbing liquid asscrubbing water to the Venturi scrubber (4).
 12. A process according toclaim 3, further comprising withdrawing some of the scrubbing liquid(10) from said different scrubbing step and passing said scrubbingliquid as scrubbing water to the Venturi scrubber (4).
 13. A processaccording to claim 4, further comprising withdrawing some of thescrubbing liquid (10) from said different scrubbing step and passingsaid scrubbing liquid as scrubbing water to the Venturi scrubber (4).14. A process according to claim 10, further comprising withdrawing someof the scrubbing liquid (10) from said different scrubbing step andpassing said scrubbing liquid as scrubbing water to the Venturi scrubber(4).
 15. A process according to claim 10, wherein in said furtherscrubbing step comprises scrubbing the gaseous impurities out in ascrubbing column (5).
 16. A process according to claim 10, wherein thegaseous impurities are scrubbed out in a packed column (5).
 17. Aprocess according to claim 15, further comprising withdrawing some ofthe scrubbing liquid (10) from said different scrubbing step and passingsaid scrubbing liquid as scrubbing water to the Venturi scrubber (4).18. A process according to claim 16, further comprising withdrawing someof the scrubbing liquid (10) from said different scrubbing step andpassing said scrubbing liquid as scrubbing water to the Venturi scrubber(4).
 19. A process according to claim 18, wherein the nozzle crosssection of the Venturi scrubber (4) is adjustable and further comprisingadjusting the nozzle cross section so as to match the amount of wastegas to be cleaned to the desired degree of cleaning.